The present invention is directed to a display apparatus which employs a digital micromirror device.
A digital micromirror device is a spatial light modulator which employs an array of thin mirrors, or micromirrors, whose positions can be electrically controlled in order to display an image. This technology has been developed extensively by Larry J. Hornbeck and his colleagues at Texas Instruments, Inc. of Dallas, Tex., and is described by them in a sequence of patents going back more than a decade. These developmental efforts have culminated in a digital micromirror device which includes an array of memory cells and a corresponding array of pivotable micromirrors whose positions are electrostatically adjusted by the contents of the memory cells. As s perhaps best described in U.S. Pat. No. 5,096,279 to Hornbeck et al, the array of pivotable micromirrors that cooperates with the memory cells can be made using integrated circuit fabrication techniques.
As is described in the above-identified patent, in U.S. Pat. No. 5,280,277 to Hornbeck, and in an article entitled xe2x80x9cMirrors on a Chipxe2x80x9d that was published in the November, 1993 issue of IEEE Spectrum at pages 27-31 by Jack M. Younse, a negative biasing voltage is selectively applied to the micromirrors and to landing electrodes fabricated beneath them in order to obtain bistable operation of the micromirrors and simultaneous updating of the entire array of micromirrors. Sometimes the micromirrors get stuck It is known that this problem can be cured by subjecting the micromirrors to resonant reset pulses which electrostatically dislodge any stuck micromirrors.
It is also known to make a color display using a single digital micromirror device by sequentially exposing it to red, green, and blue light impinging from a single direction. A white lamp and a color wheel can be employed for this purpose. If three digital micromirror devices can be devoted to a display, each of them can be illuminated by light of a different primary color and the resulting red, green and blue images can be superimposed on a screen.
Advances have also been made in other types of display apparatuses. For example U.S. Pat. No. 5,122,791 to David J. Gibbons et al discloses a ferroelectric LCD panel which is selectively backlit by red, green, and blue fluorescent tubes. The intensity or duration of the backlighting is controlled on the basis of the rank of the bits that are being displayed on the panel.
A primary object of the invention is to provide an improved display apparatus which employs only one digital micromirror device.
Another object of the invention is to provide a display apparatus in which a digital micromirror device is exposed to light at different binary levels.
Another object of the invention is to provide novel techniques for resetting a digital micromirror device so as to dislodge any stuck micromirrors.
Yet another object of the invention is to provide a display apparatus which employs a digital micromirror device wherein the micromirrors are not updated all at once, but are instead updated on a row-by-row basis while being exposed steadily to light.
In accordance with one aspect of the invention, a display apparatus includes a digital micromirror device having an array of movable micromirrors, along with exposing means for exposing them to light of a first primary color which impinges on the array from a first direction and to light of a second primary color which impinges on the array from a second direction. The first and second directions may lie in a common plane, which permits a micromirror that is ON with respect to the first primary color and OFF with respect to the second primary color when it (the micromirror) is in one of two positions to be OFF with respect to the first primary color and ON with respect to the second primary color when it (the micromirror) is in the other of the two positions. In this situation the light of the first and second primary colors impinges on the array at different times, possibly in sequences of flashes having different binary levels.
In accordance with a further aspect of the invention, a display apparatus includes a digital micromirror device having an array of movable micromirrors, along with resetting means for dislodging any micromirrors that become stuck, the resetting means including means for exposing the array of micromirrors to a magnetic field. Current through the array of micromirrors interacts with the magnetic field to jostle the micromirrors. A related aspect of the invention provides that a resetting means for dislodging any micromirrors that become stuck may include piezoelectric material beneath the micromirrors.
In accordance with a further aspect of the invention, a method for displaying a sequence of frames of video information on a digital micromirror device is provided. The digital micromirror device has an array of micromirrors that are disposed in rows and that are movable between a first position and a second position. The video information for a frame includes a plurality of first multi-bit video words (such as multi-bit video words for the red component of an image), and each micromirror corresponds to one of the first multi-bit video words. Furthermore, each of the first multi-bit video words includes at least a most significant bit and a least significant bit. The displaying method includes the step of moving micromirrors which correspond to first video words whose least significant bit has a predetermined value from their first positions to their second positions, the micromirrors of a first one of the rows being moved before the micromirrors of a last one of the rows. This is followed by the step of returning the micromirrors that were moved during the first step to their first positions, the micromirrors of the first row being returned before the micromirrors of the last row. The display method also includes the step of steadily exposing the micromirrors to light at a first level while the first step is conducted and while the second step is conducted. If the micromirrors are activated on a row-by-row basis when the first and second steps are performed and if the first and second steps are conducted at the same rate, each micromirror that is moved from its first position to its second position and then back to its first position receives the same amount of light while in the second position, regardless of the micromirror""s row. Consequently the micromirrors need not all be moved at once despite the steady illumination. It is noted that the micromirrors need not all be moved at once if they are illuminated in discrete flashes, either, instead of by steady exposure in accordance with this aspect of the invention.